Marine propulsion systems, particularly those having a lower gearcase and a submerged propeller, are subject to steering torque. Steering torque results from the rotating propeller creating a torque that tends to force the boat into a turn when the strut is aligned parallel to the desired direction of boat travel. For example, with a right hand rotating propeller, the steering torque tends to direct the boat into a turn toward port. To counteract steering torque, the strut must be directed toward port to create a slight starboard turn which compensates for the steering torque and keeps the boat traveling along the desired path. Therefore, when the boat is traveling along a straight path, the submerged strut is positioned at a slight angle, called the "crab angle", with respect to the boat direction.
When the strut is no longer positioned parallel to the direction of the boat, a pressure differential develops between one side of the strut and the other side of the strut. On the low pressure side of the strut, which is the side turned to face downstream, the flow of the water over the relatively smooth side surface is generally laminar at low speeds. As the speed of the boat increases, the flow of the water over the low pressure side surface of the strut makes a dramatic and essentially instantaneous change from a laminar flow to a turbulent flow. The instantaneous change from laminar to turbulent flow further reduces the pressure on the low pressure side of the strut, which results in the driver of the boat feeling a sudden jerk or shock. The physical location on the strut at which the water separates from laminar flow to turbulent flow is located slightly above the torpedo gearcase on the aft end of the strut.
Therefore, it can be appreciated that a strut in a marine propulsion system having a device to control the separation of water from laminar flow to turbulent flow along the strut would be desirable. Specifically, a device which enables a gradual shift from laminar flow to turbulent flow would be particularly desirable.